The invention relates to a seat belt retractor comprising an extension limiter.
Belt refractors for a seat belt are generally known. Typically they include a force limiter which, in the case of restraint, releases a defined length of webbing despite the belt reel being blocked per se so as to limit the forces acting on a vehicle occupant. The force limiter can be in the form of a torsion rod, for example, which enables the belt reel to continue rotating to a certain extent despite unilateral blocking. This reduces the risk of injury of a vehicle occupant, as in the case of a belt retractor without force limiter high forces can act on the chest of a vehicle occupant, for example, in the event of crash.
However, if is desirable in general to limit the webbing extension to a predetermined degree so that, on the one hand, the forward movement of the occupant especially on the backseats is limited to a defined extent and, on the other hand, the complete safety system of a vehicle can be optimally exploited especially with an air bag. For this purpose, in prior art usually extension limiters are employed which are functionally arranged between the belt reel and the frame. The extension limiter is independent of the force limiter and is activated by an activating mechanism as soon as deceleration loads are occurring in the case of which the force limiter reacts. The extension limiter is activated via a relative rotation between a locking member and the belt reel, wherein a thread mechanism allowing a defined screwing motion of the thread mechanism and locking further extension after passing the maximum admissible revolutions is activated by the relative rotation. The thread mechanism typically consists of two parts, for example two rings one of which has an external toothing and the other has an internal toothing. One part is rotationally fixed relative to the frame, and the other part is connected to be rotationally fixed to the belt reel. Since the force limiter allows twisting of the belt reel despite of a unilateral blocking of the belt reel, also the one part of the thread mechanism is rotating. This entails relative rotation of both parts, causing one part to move along the thread pitch. The allowed extension of the webbing permitted by the extension limiter is defined by a number of revolutions of the two parts relative to each other. After the afore-defined maximally allowed revolutions further extension of the webbing is blocked. In this way, the belt reel is blocked completely, i.e. on both sides of the belt reel, and no further extension via the force limiter is possible.
It has turned out to be a drawback in the belt retractors known from the state of the art that the extension limiters can be activated irrespective of the magnitude of the load in the case of restraint. The introduced load frequently is not so high that the extension limiter has to exercise its function limiting the extension of the webbing. Rather, the force limiter requires only a fraction of the distance made available by the extension limiter. This means that the thread mechanism of the extension limiter has performed e.g. only one of its four revolutions associated until limitation. This entails the fact that in a later case of restraint exhibiting definitely higher loads than the first one the extension limiter reacts earlier as the associated revolutions of the extension limiter were partly performed by the first trigger case. In the example then only three revolutions instead of the provided four revolutions would be available, as already one revolution was performed by the first restraining case at low load. Thus, in the second trigger case the limitation of the extension limiter takes effect earlier than provided.